Recently, the demand for lighting fixtures and the like composed of a light-emitting diode (LED) is explosively increasing due to advantages thereof, such as a longer lifespan than a conventional incandescent lamp or fluorescent lamp, relatively low power consumption, no pollutants released during a manufacturing process thereof, and the like. An LED is used not only in a display device involving light emission but also as a backlight device of a lighting fixture or a liquid crystal display (LCD) device, that is, application fields thereof are being diversified.
Particularly, since an LED can be operated at relatively low voltage and has high energy efficiency, little heat is emitted therefrom and the lifespan thereof is long. Also, with the development of technology for providing white light with high brightness, which is difficult to conventionally implement, LEDs are expected to replace most currently used light sources.
A general nitride semiconductor light-emitting device has a structure including a buffer layer, an n-type nitride semiconductor layer, an active layer, which has a multi-quantum well structure, and a p-type nitride semiconductor layer sequentially formed on a substrate, and the p-type nitride semiconductor layer and the active layer have a structure in which an upper surface of the n-type nitride semiconductor layer is partially exposed by removing partial regions of the p-type nitride semiconductor layer and the active layer through a process such as etching.
After an n-type electrode is formed on the exposed n-type nitride semiconductor layer and a transparent electrode layer is formed on the p-type nitride semiconductor layer to form an ohmic contact, a p-type bonding electrode is formed.
Meanwhile, a new attempt is being made to apply a recent LED light source to various industries over the existing range of lighting fixtures. In particular, a variety of application results are being reported from a low-power flexible display device, a wearable and attachable information display device, the field of photonic textiles in which a conductive fiber and an LED light source are combined, a body-attachable and implantable medical device, a biofusion field for verifying optogenetic validity, head mounted display (HMD) and wireless communication fields, and the like.
Generally, when an LED chip is manufactured to be small, it is possible to overcome a problem of an inorganic material being broken when bent due to features thereof. Also, by transferring the LED chip onto a flexible substrate, it is possible to impart flexibility to the LED chip and widely use the LED chip in the aforementioned various fields.
Meanwhile, it is necessary to develop a subminiature thin flexible LED light source based on a flexible device or material so that LED light sources are applied to the aforementioned various fields.
To implement such a subminiature flexible LED light source, it is necessary for a process of separating a thin film of GaN LEDs, which are formed at a size of 1 to 100 μm on a sapphire substrate, from a mother substrate through laser lift off (LLO) and transferring GaN LED structures of the separated thin film onto a flexible substrate separately or in a desired arbitrary arrangement.
Korean Unexamined Patent Publication No. 10-2014-0103278 (title: Method of Fabricating Micro-device Transfer Head) discloses a technology for placing a transfer head on a carrier substrate on which an array of micro LED structures are disposed, performing an operation of causing a phase change of an adhesive layer for at least one of the micro LED structures, picking up a micro P-N diode, a metallization layer, and optionally a part of a conformal dielectric barrier layer for at least one of the micro LED structures by using an electrostatic transfer head, and disposing the micro P-N diode, the metallization layer, and optionally the part of the conformal dielectric barrier layer for at least one of the micro LED structures on a receiving substrate.
Here, a method of attaching LEDs to a separate support substrate, inducing a phase change of an interface from solid to liquid by heating the interface, and then transferring the LEDs to individual chips by weakening adhesive strength of the interface is used to transfer each individual micro LED structure or an array of micro LED structures.
However, such a conventional technology causes a uniformity problem in a process of changing a bonding interface of at least one micro LED structure from a solid state to a liquid state, and thus some LED structures are not transferred to a pickup head.
Also, since LEDs are not separated from the support substrate, it is difficult to transfer the LEDs separately or in units of arrays. Further, an electrostatic manner of the transfer head causes serious physical damage to micro LED structures, which are very thin, which leads to a reduction in reliability.